Such machines, referred herein as "three-dimensional machines", are generally constituted by a support and a deformable arm made up of hinged arm segments, with a contact mounted at the end of the last hinged arm segment. A plurality of points on the object to be measured are then touched by means of the sensor which is movable relative to a reference coordinate system. The measurement machine is also connected to a computer which makes use of the information provided by the sensor and by position detectors associated with the arm.
The present trend is towards machines that are simultaneously compact, lightweight, and accurate. This poses problems that are particularly difficult when measuring the positions of objects inside motor vehicle bodywork.
The machines that were designed about 20 years ago are bulky and expensive (see for example documents U.S. Pat. Nos. 3,774,312, 3,774,311, 3,636,635, and 3,279,079). Furthermore, those machines are of limited application since the size of the machine determines the size of the objects that can be measured, and the cost of the machine increases rapidly with size. Because of difficulties in maintaining positioning and displacement accuracy for elements that are cantilevered out, machines of that type must also be massive throughout to ensure that they are of sufficient stiffness, such that measuring objects of large dimensions (in length or in height) requires the use of machines that are bulky, heavy, and expensive.
Machines that are simpler and easier to handle have been proposed, as illustrated in documents U.S. Pat. No. 3,944,798 and GB-A-1 498 009, or more recently in document FR-A-2 597 969. However those more recent machines are de facto more restricted as to accessible length: measurable volumes are generally limited to about 1 meter from the support of the deformable arm. As a result, if it is absolutely essential to perform measurements in a zone that is further away, then the support must be moved, and that gives rise to a loss of accuracy since the computer determines the coordinates of the contact sensor in a frame of reference based on the machine.
It might be tempting to use a guide rail and to mount the support so as to be free to move in translation on the rail. However, if it is desired to maintain high accuracy (to within two-tenths of a millimeter), then the rail must be of a section that is large enough to avoid twisting, and must therefore be heavy and bulky.